This invention relates generally to a web tracking apparatus, and more particularly to apparatus for controlling the alignment, in a lateral (cross-track) direction, of a web moving along a path.
In high speed electrographic reproduction apparatus for example, it is a common practice to employ an elongated dielectric belt or web adapted to carry transferable images and moving in a path in operative relation with electrographic process stations. Typically the web is supported by, and driven about, at least one roller. With a roller support, there is a tendency for the moving web to shift laterally with respect to such roller. Various apparatus for correcting for such lateral (cross-track) shifting of roller-supported webs are known, such as crowned rollers, flanged rollers, servo actuated steering rollers, or self-activated steering rollers for example. However, crowned rollers are not suitable for use with a web in an electrographic reproduction apparatus because they force the web toward the apex of such rollers and cause distortion of the web and produce local stresses in the web at the crown which can damage the web. Flanged rollers are also not suitable because they produce a concentrated loading at the edges of the web resulting in edge buckling, seam splitting, or excessive edge wear.
Electrographic reproduction apparatus therefore typically utilize servo actuated or self-activated steering rollers. While such steering rollers will correct for cross-track shifting of the web, they tend to produce significant back-and-forth lateral movement of the web as it is realigned. In making monochromatic reproductions the lateral movement, within limits of course, is not critical since only one discrete area of the web is used in generating any one reproduction. However, when either composite monochromatic or multi-color reproductions are being made, the degree of lateral movement becomes a significant limiting factor in obtaining quality output from the reproduction apparatus.
For example, in making multi-color reproductions with an apparatus utilizing a moving dielectric web, charge patterns corresponding to related color separation images of input information are formed in successive discrete areas of the web. Such patterns are developed with appropriate pigmented marking particles to form transferable images which are transferred sequentially to a receiver member to form the multicolor reproduction. The sequential image transfer must take place in accurate superimposed register in order to obtain quality output (i.e., faithful multi-color reproduction). Therefore, lateral movement of the web must be minimized so that lateral deviation between successive discrete areas is within acceptable limits whereby transferable images formed at such successive discrete areas are alignable in accurate superimposed register at transfer. Known servo actuated or self-activated steering rollers react to the absolute lateral position of the edge of the web and, since such edge may not be true, can cause formation of sequential transferable images on discrete areas outside the acceptable limits which allow accurate superimposed register at transfer.